In order to prevent malfunction or destruction of a microprocessor, it is necessary to limit the signals at the signal and voltage inputs of the microprocessor.
A measure known by companies for limiting input signals is known as signal “clamping”. For this purpose an input of the microprocessor, as shown in FIG. 2, is connected via a first diode (D1) to the voltage supply (VCC) to the microprocessor. The anode of the diode is connected to the input. The anode of a second diode (D2), the cathode of which is connected to the input, is earthed. This means that the maximum input voltage is equal to the sum of the supply voltage and a threshold voltage of the respective diode. The electric current flowing into the input of the microprocessor is limited by means of a resistor.
One disadvantage of this circuit arrangement is that the supply voltage to the microprocessor increases, once the latter is in an inactive mode and can no longer consume the current fed in via the protective circuit.
Companies also know how to supplement this circuit arrangement with a Zener diode (D3), which is connected between earth and the supply voltage (VCC)—as shown by the broken line in FIG. 2. The Zener diode (D3) becomes conductive once a threshold voltage is exceeded. This prevents a build-up of supply voltage (VCC).
Because of the frequently strict voltage tolerances of a microprocessor, it is however usually not possible to use a Zener diode (D3), as the predetermined voltage thresholds mean that there is either a significant idle current or the voltage is no longer limited within a permitted operating voltage range.
FIG. 3 shows a third protective circuit for a microprocessor known to companies. With this circuit arrangement a transistor stage is connected before each microprocessor input; this transistor stage, comprising a transistor (T5), a diode (D5) and three resistors (R5, R6 and R7), has the function of potential isolation. The transistor stage means that the voltage at the input of the microprocessor is never greater than the supply voltage at the microprocessor.
It is a disadvantage here that the connections to every input mean that a significantly larger number of components have to be used and every transistor in turn has to be protected against overvoltage.
A method and device for protecting one or more vehicle control devices connected to a motor vehicle on-board network against overvoltage pulses are known (DE 197 42 391 C1). With these, a sensor is used to detect an overvoltage pulse and one or more consumer units are enabled when such overvoltage occurs. In this way the overvoltage pulse is degraded quickly and before it reaches its maximum value and the electronic circuits and control devices in the motor vehicle are therefore protected. With the known device external loads, such as dipped headlights, an interior light, air conditioning unit, regulating motor or central locking are activated, to compensate for an overvoltage pulse.